The present invention is directed to an apparatus and related method for obtaining an azimuthally directed measurement in a cased well borehole and more particularly in one provided with a production tubing surrounded by a gravel pack on the exterior of the production tubing and on the interior of the casing. It is particularly useful for wells into formations which are produced in this fashion, namely, by positioning a casing in the well borehole, cementing the casing in the well and subsequently forming perforations through the casing into the formation so that formation fluid production is obtained. In many wells, one problem is that there will be excessive sand production from a producing formation, and that is often countered by installing a gravel pack in the cased well. A typical arrangement involves a production tubing string centralized within a casing cemented in place with a gravel pack and sand screen supporting the gravel pack on the interior of the casing.
Gravel packing is performed to keep loosely compacted formations from eroding during production. Formation on erosion generally begins at or near the perforation tunnels where fluid flow velocities are highest. When this type of erosion occurs, there are several possible detrimental results such as formation fines which plug the formation and reduce or stop production; they may fill the casing stopping production, and they may be carried by the production stream where they can cause a variety of equipment damage.
The idea behind gravel packing is to fill the perforation tunnel with a permeable material which reduces the flow velocity. It is also desirable that this packing be of a similar pore size to the formation in order to further reduce the movement of formation fines. In the event that the perforation tunnel portion of the gravel pack is not completely successful, the annular portion of the pack inside the casing may act as a barrier to filter the formation material from being carried downstream by the fluid flow.
It is very difficult to measure how well the perforation tunnels are packed. However, much can be learned about the quality of the packing procedure by measurements which detect the uniformity of the annular portion of the pack inside the casing. It is desirable to detect both increases and decreases in gravel pack porosity which may indicate voids and plugging respectively.
Immediately after performing a gravel pack procedure, before flowing the well, voids in the annular area and inside the casing may indicate that the perforation tunnels were not sufficiently packed. It also, belies later problems in that even if the tunnels are well packed, the annular void provides a location for the flowing fluid to carry the pack material from the perforation tunnel into the casing thereby unpacking the tunnel. After a well is produced and there is a partial failure of the packing (some or many tunnels are not packed), the annular portion of the pack acts as a filter to prevent formation fines from moving downstream. Voids detected at this time indicate the reduced capability of this filtering material. This type of failure may also be indicated by the reduced packing porosity, production fluids into the casing at high flow velocities, they will actually erode the gravel pack screen itself if it is not protected by the annular portion of the pack. A work over is necessitated to correct the pack. Work overs interrupt production and cost substantial sums of money to provide service to a well. Even then, when the work over is complete, the pack in the well may sand up again.
In conjunction with gravel pack, a screen typically will be installed, namely a screen formed of screen wire or screen cloth which is inserted in the well borehole to prop up the gravel pack. This defines an annular support for the gravel. This is highly desirable to extend the life of a well.
It is possible to locate a void in the gravel with a tool which is responsive to density. Consider for instance a density measuring device where there is a substantial contrast between the fluid in the pores and the gravel. The fluid may have a density of about 1.0, but perhaps slightly more if it is salt water, and the gravel pack material might have a density of about 2.65 gm/cc. A loss of gravel pack material in a particular region will alter the matrix/fluid ratio and thus reduce the measured bulk density. Conversely, a pack plugging with formation fines will have an increased density. The density is inversely proportional to the detected count rate of a typical gamma ray fluid density tool used in this circumstance and can be employed to indicate gravel pack quality.
That type measurement is made all the more difficult as a result of recent advances which have been introduced for gravel pack materials. The contrast in the density of the matrix and fluid has been reduced with the advent of new packing materials. Regrettably, this makes measuring gravel pack quality more difficult. In other words, as the specific density of the matrix material decreases from a typical density of 2.65 down to 2.0 or perhaps even less, the loss in contrast in the density measurement between the matrix material and the pore fluid makes measurement the gamma density approach difficult, perhaps almost impossible. The present disclosure sets forth a method and apparatus which can be used to measure gravel pack quality that does not depend on a contrast between the fluid and matrix material densities.